Dynamic 2D Array Initialization

Question

I want to create and initialize a 2D array but the initialization is failed. I encounter "The program has unexpectedly finished." in Qt Creator.

What is wrong?

in .h file

private:
    int pop;
    int d;
    float **uye;

in .cpp file

pop=50;
d=12;
uye = new float*[pop];
for(int i=0; i<d; i++) uye[i] = new float[d];

for(int n=0; n<pop; n++)
{
    for(int m=0; m<d; m++)
    {
        uye[n][m] = (float) n*m;
    }
}

A 2D jagged array with dimensions determined at runtime that doesn't leak: std::vector<std::vector<float>>. — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Dec 25, 2011 at 11:49

fredoverflow · Accepted Answer · 2011-12-25 11:55:32Z

3

The first loop for(int i=0; i<d; i++) should probably be for(int i=0; i<pop; i++). Otherwise, you are only reserving space for 12 elements, but later try to access 50.

Note that having raw pointer members is considered a very bad idea in modern C++, because you need to worry about copying semantics. Better use a flat std::vector<float> and do the 2D to 1D mapping manually, or use a std::vector<std::vector<float> > with convenient access syntax.

I would prefer the second version. Without seeing more context:

pop = 50;
d = 12;
uye = std::vector<std::vector<float> >(pop, std::vector<float>(d));

The nested for loops that follow work exactly the same, no changes required.

edited Dec 25, 2011 at 11:55

answered Dec 25, 2011 at 11:50

fredoverflow

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Comments

Xeo · Accepted Answer · 2011-12-25 11:51:41Z

3

What is wrong?

You're not using std::vector (that's one of the things that's wrong, @FredO covered the other thing).

#include <vector>

int main(){
  typedef std::vector<float> inner_vec;
  typedef std::vector<inner_vec> outer_vec;

  int pop = 50, d = 12;
  // first parameter to vector is its size
  // second is what every element should be initialized to
  outer_vec uye(pop, inner_vec(d));
  for(unsigned n = 0; n < uye.size(); ++n){
    for(unsigned m = 0; m < uye[n].size(); ++m){
      uye[n][m] = (float)n*m;
    }
  }
}

answered Dec 25, 2011 at 11:51

Xeo

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